Re: The Categorical Imperative
I hope this makes more sense than the dialectic of can and must.
Kant based his philosophy on two necessary truths, the starry heavens above and the moral law within. His theory of morality, the Categorical Imperative, says it is necessary to do our duty, as a single ultimate commandment of moral reason. Kant said the categorical imperative is that the highest principle of moral duty is that we should treat people as ends not means, meaning we should engage with people for their own intrinsic value rather than to exploit them. As DWill pointed out, this is the golden rule of Christianity and other traditions, treat others how you would like them to treat you, dressed up in philosophical jargon.
Kant explained that his reasoning was based on the need to respond to the erroneous skepticism of David Hume, primarily around causality. In morality, Kant aims to show the error of Hume's arguments that reason is slave of passion and that morality is based on intuition rather than logic.
Haidt's presentation of logic versus empathy as a zero sum game (per DWill's summary above) on face value illustrates a major failure of ethical understanding on his part. Systematic logic is actually at the basis of genuine empathy, because logic looks past the immediate moment to set the situation in the framework of subsequent consequences, and empathises more deeply than immediate sentiment.
The argument that logic and empathy are in conflict is readily refuted by any situation where we have to do something unpleasant in order to achieve a higher good, as in the maxim no pain no gain. Discipline and vision are the basis of achievement, as a matter of logic. If a coach empathises with the desire of an athlete not to train, the coach is actually failing to support the deeper interests of the athlete, and putting passion before reason. Similarly when a teacher empathises with a student who does not want to work to their potential, they commit a failure of logic, a failure to follow through on the rational knowledge that the consequence of empathising with laziness is to produce poor results.
Kant's way of thinking bears some resemblance to the scientific method of the anthropic principle, which starts from the observation that we exist and asks what the universe must be like in order that our existence occurs and makes sense. For example, logic tells us that for our lives to make sense the universe is self-consistent, that two contradictory statements cannot both be true, and this is a necessary truth of reason and experience. So Kant seeks to systematically deduce the necessary conditions of experience to define the foundational axioms of true philosophy, as an exercise in transcendental imagination. Kant argued for what he called synthetic a priori judgments, or necessary truths. For example, time, space and causation must exist, because without them our experience would not be possible.
I have always wondered about the logical basis of Kant's categorical imperative. Why is it necessary to treat people as ends? It is a very metaphysical idea, and quite obscure. Adam Smith argued that we routinely treat people as means in economic relations, and that the invisible hand of the market means that this optimises well being. As a synthetic a priori morality, I prefer Heidegger's axiom that care is the meaning of being. This is based on Kant, but it seems to me it more rigorously starts from real experience, pointing out that without care there is no meaning in life, and that the network of relations built upon care provides a foundation for moral understanding. Heidegger undermined his moral philosophy with his unwise political commitments, but care still stands as a simpler and clearer framework than Kant's kingdom of ends.
Heidegger also opened up some of the intuitive domain of philosophy, but in a way that was clearer than the little of Haidt I have read. For example Heidegger criticised the Kantian theory of reason for failing to incorporate moods such as angst (anxiety or dread), and argued that such existential phenomena as angst are in fact at the basis of care.